Genetic variation in populations and its implications

Genetic variations exist in virtually all forest organisms; they are particularly important in tree species and in the pests and pathogens that attack them. The genetic basis of forest pathology and its influence right up to the landscape level was the basis of the 1999 Montreal symposium of the American Phytopathological Society. Much of what was discussed then has been updated (Lundquist and Hamelin, 2005) in a volume that emphasizes how rapidly long-lived trees and forests can be devastated by pathogens and microscopic organisms such as fungi, bacteria, phytoplasmas and viruses. Pathogens have important effects on biodiversity, greatly influencing plant populations of both natural forests and plantations. The numbers of important indigenous species may be greatly reduced; some may be eliminated altogether. On the other hand pathogens themselves contribute to the biological diversity of the ecosystems they inhabit.

Severe devastation of the cypress Cupressus lusitanica by the cypress aphid Cinaria cupressi in East Africa appears to have been facilitated by the narrow genetic base of the planting stock. Similarly, Dutch elm disease in the UK (Section 5.4.5) was particularly bad because many of the 25 million trees killed were English elm Ulmus procera that all derived from a single clone brought to the UK from Italy by the Romans 2000 years ago for use in supporting and training grape vines (Gil et al., 2004).